In a conventional engine-driven inverter power generator, a generator output is decided based on an engine output and an efficiency of alternator and inverter. In other words, the generator output supplied to a load is limited by the engine output. There is known a hybrid engine-driven power generator in which a battery as auxiliary power supply is provided and an output voltage of the battery is added to an output voltage of an alternator via a DC/DC converter to supply a generator output having the engine output (power) or more to the load (see Japanese Patent Publication No. 3941927). In the hybrid engine-driven power generator, when the engine frequency reduces due to an overload state so that the generator output of the alternator reduces, the battery voltage is boosted by the DC/DC converter to assist the output of the alternator by the boosted voltage.